This invention relates to pipe couplings, and specifically to methods for coupling pipes using a sleeve that is connected to the pipe sections by an interference fit.
The concept of joining mechanical elements by an interference fit is well known, and probably has historical antecedents dating back to the last century. The joining of tubular members by interference fit means is disclosed at least as early as British patent specification No. 435,700 (Mannesmann) accepted on Sept. 16, 1935.
Normally, an interference fit coupling is made by selecting one tubular element having a given outside diameter, and a second element, also tubular, having an inside diameter slightly less than the outside diameter of the first element. By mechanical means, the two elements are forced together distending the outer element. Generally, the inner element is radially compressed, and the outside element is radially distended by a given value determined by the relative inside and outside diameters.
In the above noted British specification, the inventor teaches a method of joining tubular and like members having male and female parts, comprising the step of forcing together the two members to such an extent that one or both of the members is stressed beyond its elastic limit, with a resulting permanent alteration in the shape or size of the stressed element. According to the patentee, the joints made according to the method disclosed in the British patent specification have excellent cohesive strength even when tested to destruction. However, the creation of couplings by the above means did not provide a joint capable of withstanding internal pressures, as would occur, for example, in a gas or petroleum pipeline. There remained a need for a joint that could be fabricated reliably in the field and would withstand the high internal pressures normal in pipelines without leakage or destruction.
Later attempts were made to improve the interference fit joining of pipelines and one such method is taught in Canadian Pat. No. 1,125,811, granted on June 15, 1982 (Hauk, et al).
Hauk et al provide a joint comprising a sleeve having a radially inwardly directed flange. The flange is disposed centrally in the sleeve, and pipe sections having an outside diameter greater than the inside diameter of the sleeve are forced into the sleeve by hydraulic or other means to engage the flange, there being disposed between the flange and the ends of the pipe sections gasket material intended to seal the ends of the pipe sections to the sleeve. However, Hauk et al clearly stated that the sleeve should not be stretched beyond its elastic limit. Hauk et al, therefore, appeared to be teaching that contrary to the teachings of the British patent No. 435,700 the stretching of the sleeve beyond its elastic limit was detrimental to the joint ultimately achieved.
Others have addressed the same problem in different ways. For example, Gibson in U.S. Pat. No. 4,328,983 granted on May 11, 1982 teaches a method similar to the methods taught in the specifications mentioned above, however, a sealing substance is applied to the marginal ends of the pipe prior to joining and the coupling member or sleeve is internally grooved so that the grooves engage the outside of the pipe, which is intended to provide a stronger joint. The interference fit concept forms part of the teachings of Gibson. Many others have attempted to provide interference fit joints but success has been limited in that no joining method or construction has achieved sufficiently high reliability, or ease of installation in the field, to be accepted by the pipeline industry.